There is a notch filter referred to as an N-path filter (N-path notch filter), for example, as the notch filter; this is used for eliminating a blocker and the like in wireless communication, for example.
That is to say, the notch filter is used for eliminating an interferer/blocker of 13.56 MHz emitted by a near field communication (NFC) device in a receiver of a body area network (BAN), for example.
Herein, the BAN is a new communication standard for which international standards such as IEEE 802.15.6 and ECMA-401 are issued recently. The BAN is assumed to be applied to short-distance non-contact wireless communication between a wearable device mounted on a human body and a portable terminal such as a cell phone with low power consumption, for example.
Also, NFC being non-contact wireless communication at a close distance of few centimeters is standardized as ISO/IEC 18092 (NFCIP-1). The NFC is used in an electronic commuter pass, electronic money and the like, for example, and a portable terminal such as a cell phone having a function of such electronic commuter pass and electronic money is widespread.
Therefore, it is supposed that the portable terminal performs both BAN and NFC communication.
Although communication is performed at approximately 40 Mbps at a maximum by using a minute induced electric field of a wide range of frequency band from few MHz to several tens of MHz in the BAN, data communication is performed at a relatively low speed of 424 kbps at a maximum by using a relatively narrow frequency band of approximately 1 MHz in the vicinity of 13.56 MHz in the NFC.
Since the communication is performed while power is supplied by using the induced magnetic field in the NFC, transmission power becomes larger as compared to that in the BAN, so that a notch filter which prevents an interferer/blocker from the NFC is required in the receiver of BAN.
It is confirmed that the interferer/blocker from the NFC is stronger than the signal of the BAN by approximately 30 dB by experiment; both the BAN and the NFC have frequency deviation of ±50 ppm at a maximum. Therefore, the notch filter is requested to correctly adjust a central frequency thereof to 13.56 MHz and eliminate a range of approximately ±100 ppm around the same with an attenuation amount of 30 dB or larger.
Realization of the notch filter which eliminates such a correct frequency band is difficult by using a continuous-time analog circuit in which the central frequency easily changes due to variation of an element constant, so that it is considered to realize the same by using a discrete-time analog circuit operating with a clock.
There is an N-path filter, for example, as a notch filter which eliminates a narrow-band blocker by using the discrete-time analog circuit (for example, refer to Non-Patent Document 1).